"Companies should respect Iraqi laws and the dignity of the citizens," he added.

The BBC's Hugh Sykes in Baghdad says the status of the thousands of often heavily armed private security guards employed in Iraq is unclear.

The guards are considered neither civilians nor military personnel, although they do carry IDs from the US Department of Defense.

Order 17 of the Coalition Provisional Authority gives the guards immunity from Iraqi prosecution, but they have no combat immunity under international law if they engage in hostilities.

Any Iraqi review of their status would therefore only have an effect if the US authorities accept its conclusions, our correspondent says.

'Fair probe'

Earlier, the US Secretary of State, Condoleezza Rice, telephoned Prime Minister Nouri Maliki to express her regret over the deaths and pledge to help carry out a "fair and transparent" investigation into the incident.

A spokesman for Ms Rice told the AFP news agency that she had "reiterated that the United States does everything it can to avoid such loss of life in contrast to the enemies of the Iraqi people who deliberately target civilians".

The two agreed to hold any wrongdoers accountable, according to Mr Maliki's spokesman.

Blackwater is one of the biggest private security contractors in Iraq and is reported to have a contract worth $300m (£150m) with the state department to protect its diplomatic staff and equipment there.

Last week, the US ambassador to Iraq, Ryan Crocker, spoke of the importance of private security contractors in Iraq and correspondents say their suspension would be a potentially serious blow to the state department's work there.